Method of purifying pulp



Jan. 15, 1935 Filed Sept. 20, 1953 \NVENTORS:

H-F'REEMAN and JAM HUNTER ma i/5.

' ATTORN Ey.

Patented Jan. 15, 1935 METHOD or w Horace Freeman and John WalterHunter,

Rivers, Quebec, Canada, assignors to Consolidated Paper GorpnrationLimited, Montreai,

Quebec,

Application September at, was, In C Septem 21, 193% This inventionrelates to a method of treating ground wood pulp for the removal ofimpurities. In the production of pulp from wood and the like there areusually present impurities which 5 cause discolouration in'the paperproduced from the pulp and which have other disadvantages in the pulp.These impurities consist of particles of dirt, bark, badly discolouredfibres, pitch, rosin and the like. Moreover, long fibres and bundles offibres may act as impurities in some classes of pulp and for the purposeof the description of the invention are to be so regarded herein.

The object of the present invention is to provide a method whereby theseimpurities may be readily removed from the desirable fibre leaving itfree from undesirable material. The invention contemplates changing therelative buoyancy of the particles of impurities and of fibres so thatthey may be separated when in the form of pulp.

) This change in buoyancy is made possible by the difference, which hasbeen observed, in the character of the surface of the impurities andthat of the fibre. Air bubbles are thus caused selectively to adhere tothe impurities and the fibre'to be 5 removed.

In the. production of ground wood pulp the logs of wood are comminutedagainst stones and inevitably particles of knots, rosin, bark, skin,dirt and the like are formed in such a fine state 3 of division thatthey may not be removed by the usual processes of screening. Owing tothe usually oily or resinous character of the surface of these particlesof impurities bubbles of air will cling to them. Thus, while theseparticles and 5 the wood fibre are in suspension in water, the pulp issubjected to the action of air which forms bubbles about the particlesof impurities. It has been found, particularly when the water isrecirculated to the grinders in a more or less closed cycle, that thesugars, wood oils and the like which accumulate in the water aresufficient to constitute the necessary bubble-forming agent.

This bubble-forming and flotation step maybe carried out at any stageinthe treatment of the pulp but preferably after the ground wood leavesthe grinders. The pulp may be caused to fiow through a trough or launder.with a canvas bottom through which the air is admitted to form bubblesand a top overflow trough along the sides into ,0 which the foamcarrying the impurities passes while the fibre passes on to the nextnormal step in the process. Alternatively, the'pulp may continuouslyflow from the grinders into a large tank, at the bottom of which air istied carrying a the impurities to the surface from whence it is i No.teases bottom of the bubble-forming zone in order to 5 afiord ampleopportunity for all of the particles of impurities to attach themselvesto bubbles and become sumciently buoyant to float and flow from thesurface in the separating zone. The air tends to agitate the pulp butadditional agitation w may be employed if desired to insure liberationof all the impure particles. The foam having been removed, the pulpcontains only the purified fibre which is then ready to be run intopaper or used for, any other purpose.

One form of apparatus for carrying out the method is disclosed in theaccompanying drawing, in which Figure 1 is a plan view;

Figure 2 is a section on line 2-2; and

Figure 3, a section on line 3-3.

In the drawin 1 is a trough to which the pulp is admitted by means ofthe inlet 2 and which has a porous bottom 3 forming an air chamber 4supplied with an air inlet 5. Launders or troughs 6 and d'extend alongthe sides of the main trough 1- and are provided with outlets and '7'. Adam 8 is provided adjacent the outlet end of the trough 1 to stop .theforward flow of the foam and prevent its escape withthe treated stockthrough the outlet 9 which is controlled by an adjustable weir 10.

In operation the pulp stock is' admitted to the trough 1 and air underpressure is admitted to the chamber 4. In passing through the porousbottom into the stock the air forms fine bubbles which, upon rising,selectively adhere to solid portions of the stock, making the same morebuoyant so that they rise to the surface with the foam and are carriedoff through the launders, leaving the treated stock free from theselectively removed portions. The height of the stock in the trough, thedepth of the foam accumulated on the surface, and the quantity of stockwhich may overflow into the launders are regulated by raising orlowering the weir l0.

It will be apparent that the invention may be applied at other stages inthe treatment of the pulp than those mentioned and that a great varietyof means may be employed for forming the foam and eflecting its removalfrom the frequently occasioned by the accumulation of 56 pitch givingtrouble on the screens and in other. parts of the equipment.Furthermore, the dimculties frequently experienced by the papermaker dueto the more or less accidental formation of foam are avoided.

It will be observed that the .process is a selective one, in. that'it.is' efiective in actually separating selected constituent of thepulp, leaving the remaining constituents in suspension inthe fluid orwater content of the pulp. In this respect it is comparable to theflotation process as applied to the concentration of ore. The

process has wide application in the treatment of cellulose pulp and maybe employed to assist screening operations or as a substitute therefor,particularly in the separation of long fibres and bundles of fibres.

We claim:

1. A method of purifying ground wood pulp which comprises passingnumerous finely divided streams of air upwardly through a mass of thepulp in suspension in the water circulating through the grinder andseparating the froth so formed from the pulp. v I

2. A method of treating ground wood pulp to remove,impurities therefromwhich comprises passing numerous fine streams of air upwardly in a massof said pulp containing water in which it was formed and removing fromthe surface of the pulp the froth so formed.

3. A method of purifying ground wood pulp which comprises passing finestreams of air into the pulp in substantially the condition in which itcomes from the grinders to form bubbles to which impurities in the pulpadhere and removing the froth with the adhering particles.

4. A method of purifying ground wood pulp which comprises passing finestreams of air upwardly in the pulp after it leaves the grinder andbefore washing to form bubbles, collecting said bubbles and adheringimpurities at the surface of the pulp and removing the same.

5. A methodpf treating ground wood pulp which comprises passing theunwashed pulp through atrough, admitting fine streams of air at thebottom of the trough to form with natu-.

rally occurring reagents in the pulp bubbles to which impurities in thepulp adhere and rise with the bubbles tothe surface of the pulp andremoving the bubbles and adhering impurities.

- HORACE FREEMAN.

JOHN WALTER HUNTER.

